Programmable pulse capture device with automatic gain control

ABSTRACT

The invention an optical system and a method for automatically controlling the gain of a receiver in an optical system. The optical system includes an optical receiver, a pulse capture unit, and an automatic gain control. The pulse capture unit includes a capture unit capable of capturing an optical signal received by the optical receiver; and, a process unit capable of processing the captured optical signal. The automatic gain control is capable of controlling the gain of the optical receiver responsive to the content of the processed optical signal. The method includes comparing the intensity of at least one returned pulse, and typically a plurality of returned pulses, to a predetermined value; and controlling the gain of an optical detector responsive to the comparison. In addition, the maximum gain is controlled by a noise limit in some implementations.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to laser detection and ranging (“LADAR”)systems, and, more particularly, to the pulse capture electronics ofLADAR systems.

2. Description of the Related Art

A need of great importance in some military and civilian operations isthe ability to quickly detect, locate, and/or identify objects,frequently referred to as “targets,” in a “field of view.” A commonproblem in military operations, for example, is to detect and identifytargets, such as tanks, vehicles, guns, and similar items, which havebeen camouflaged or which are operating at night or in foggy weather. Itis important in many instances to reliably distinguish between enemy andfriendly forces. As the pace of battlefield operations increases, sodoes the need for quick and accurate identification of potential targetsas friend or foe, and as a target or not.

Useful techniques for identifying targets have existed for many years.For instance, in World War II, the British developed and utilized radiodetection and ranging (“RADAR”) systems for identifying the incomingplanes of the German Luftwaffe. RADAR uses radio waves to locate objectsat great distances even in bad weather or in total darkness. Soundnavigation and ranging (“SONAR”) has found similar utility andapplication in environments where signals propagate through water, asopposed to the atmosphere. While RADAR and SONAR have proven quiteeffective in many applications, they are inherently limited by a numberof factors. For instance, RADAR is limited because it uses radiofrequency signals and large antennas used to transmit and receive suchsignals. Thus, alternative technologies have been developed anddeployed.

One such alternative technology is laser detection and ranging(“LADAR”). Similar to RADAR systems, which transmit radio waves andreceive radio waves reflected from objects, LADAR systems transmit laserbeams and receive reflections from targets. In LADAR systems, brieflaser pulses are generated and transmitted via an optical scanningmechanism. Some of the transmitted pulses strike a target and arereflected back to a receiver associated with the transmitter. The timebetween the transmission of a laser pulse and the receipt of thereflected laser pulse (a “return pulse”) is used to calculate the“range” from the target to the object that receives the return pulse.

Because LADAR provides range information, the data is“three-dimensional,” i.e., it provides information about the target inthree dimensions. Typically, these dimensions are range, azimuth, andelevation. The shorter wavelengths of light signals (relative to radiosignals) also provide much higher resolution and tighter beam control.These attributes of LADAR data greatly assist not only with targetlocation, but also target identification. Thus, in many respects, LADARsystems can provide much greater performance than can, e.g., RADAR andSONAR systems.

The evolution of one particular LADAR system can be traced by reviewingthe following issued U.S. Letters Patent:

-   -   U.S. Pat. No. 5,243,553, entitled “Gate Array Pulse Capture        Device,” issued Sep. 7, 1993, to Loral Vought Systems        Corporation as the assignee of the inventor Stuart W.        Flockencier;    -   U.S. Pat. No. 5,357,331, entitled “System for Processing        Reflected Energy Signals,” issued Oct. 18, 1994, to Loral Vought        Systems Corporation as the assignee of the inventor Stuart W.        Flockencier;    -   U.S. Pat. No. 5,511,015, entitled “Double-Accumulator        Implementation of the Convolution Function,” issued Apr. 23,        1996, to Loral Vought Systems Corporation as the assignee of the        inventor Stuart W. Flockencier; and    -   U.S. Pat. No. 6,115,113, entitled “Method for Increasing        Single-Pulse Range Resolution,” issued Sep. 5, 2000, to Lockheed        Martin Corporation as the assignee of the inventor Stuart W.        Flockencier.        These patents all describe the data acquisition electronics, or        “pulse capture electronics” (“PCE”), of the LADAR system. They        also disclose a LADAR transceiver whose operation is more fully        disclosed in the following patents, among others:    -   U.S. Pat. No. 5,200,606, entitled “Laser Radar Scanning System,”        issued Apr. 6, 1993, to LTV Missiles and Electronics Group as        the assignee of the inventors Nicholas J. Krasutsky, et al.    -   U.S. Pat. No. 5,224,109, entitled “Laser Radar Transceiver,”        issued Jun. 29, 1993, to LTV Missiles and Electronics Group as        the assignee of the inventors Nicholas J. Krasutsky, et al.; and    -   U.S. Pat. No. 5,285,461, entitled “Improved Laser Radar        Transceiver,” issued to Feb. 8, 1994, to Loral Vought Systems        Corporation as assignee of the inventors Nicholas J. Krasutsky,        et al.        This LADAR system also processes the acquired data for a number        of end uses, such as those more fully disclosed in a number of        patents, including:    -   U.S. Pat. No. 5,644,386, entitled “Visual Recognition System for        LADAR Sensors,” issued Jul. 1, 1997, to Loral Vought Systems        Corp. as assignee of the inventors Gary Kim Jenkins, et al.    -   U.S. Pat. No. 5,852,492, entitled “Fused Lasar Range/Intensity        Image Display for a Human Interpretation of Lasar Data,” issued        Dec. 22, 1998, to Lockheed Martin Vought Systems Corp. as the        assignee of the inventors Donald W. Nimblett, et al.; and    -   U.S. Pat. No. 5,893,085, entitled “Dynamic Fuzzy Logic Process        for Identifying Objects in Three-Dimensional Data,” issued Apr.        6, 1999, to Lockheed Martin Corp. as the assignee of the        inventors Ronald W. Philips, et al.        Each of these patents is commonly assigned herewith to the        assignee of this invention, Lockheed Martin Corporation.

One concern with virtually all LADAR receivers is the “gain” of theirdetectors. The gain controls the amount of amplification applied by thedetector to a return pulse when it is received. The gain should becommensurate with the intensity of the return pulse. If the intensity ofthe return pulse is high, then the gain of the detector should be low toavoid over-saturating the detector's components. On the other hand, ifthe intensity is low, the gain should be high to facilitate subsequentprocessing, although not so high that “noise” is reported as a returnpulse.

Setting the detector's gain, however, is fraught with many difficulties.The intensity of return pulses can vary wildly from one moment to thenext and creates difficulty in setting the gain for any particularintensity level. Setting the gain arbitrarily high unnecessarily risksover-saturating the detector's components for high intensity returns anderroneously reporting noise as returned pulses. On the other hand,setting the gain unnecessarily low risks missing low intensity returns.

The present invention is directed to resolving, or at least reducing,one or all of the problems mentioned above.

SUMMARY OF THE INVENTION

The invention, in its various aspects and embodiments, includes anoptical system and a method for automatically controlling the gain of areceiver in an optical system. The optical system comprises an opticalreceiver, a pulse capture unit, and an automatic gain control. The pulsecapture unit includes a capture unit capable of capturing an opticalsignal received by the optical receiver; and, a process unit capable ofprocessing the captured optical signal. The automatic gain control iscapable of controlling the gain of the optical receiver responsive tothe content of the processed optical signal. The method comprisescomparing the intensity of a returned pulse to a predetermined value;and controlling the gain of an optical detector responsive to thecomparison.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1 depicts, in a block diagram, one particular embodiment of a LADARsystem employing the present invention;

FIG. 2 conceptually illustrates the relative position of the LADARsystem of FIG. 1 to a target within a field of view during dataacquisition;

FIG. 3 is a block diagram of selected portions of the pulse captureelectronics of the LADAR system of FIG. 1;

FIG. 4 is a timing diagram for selected events occurring during dataacquisition;

FIG. 5A is a block diagram of an automatic gain control (“AGC”) for thepulse capture electronics in accordance with the present invention;

FIG. 5B illustrates one embodiment of a intensity median computationunit in the AGC in FIG. 5A;

FIG. 6 is an exploded view of several components of an optical trainthat is the LADAR transceiver of the LADAR system of FIG. 1;

FIG. 7 is a block diagram of one particular embodiment of the pulsecapture electronics of the LADAR system of FIG. 1;

FIG. 8 is a block diagram of the capture unit of the pulse captureelectronics shown in FIG. 7;

FIG. 9 and FIG. 10 are a block diagram and a timing diagram,respectively, illustrating the timing of sampling in the pulse captureelectronics of FIG. 7;

FIG. 11 is a block diagram on particular implementation of the processunit in FIG. 7;

FIG. 12 to FIG. 16 illustrate a first implementation of a convolutioncircuit of FIG. 11, wherein:

FIG. 12 is a graph of a filter coefficient stream (“FCS”) representingan example of an input signal for two matched filter implementations;

FIG. 13 is a table illustrating the convolution of the sample functionof FIG. 12 for times (t⁻¹, t₀, t₁, . . . t₆) and for each of “n”samples;

FIG. 14 and FIG. 15 comprise a block diagram and an input/output table,respectively, which illustrate a conventional high-speed circuitapproach for convoluting an input signal to the circuit; and

FIG. 16 shows 2-input adders with which a summation circuit of FIG. 14is implemented in one particular embodiment;

FIG. 17 to FIG. 31 illustrate a second implementation of the convolutioncircuit of FIG. 11 alternative to that illustrated in FIG. 12 to FIG.16, wherein:

FIG. 17 is a block diagram of a D-A convolver circuit;

FIG. 18 and FIG. 19 graph the respective first and second derivatives,respectively, of the FCS represented by FIG. 12;

FIG. 20 illustrates a set of D-A coefficients, obtained in accordancewith the present invention by taking the second derivative of the filtercoefficient stream of FIG. 12 against the axis B_(n);

FIG. 21 to FIG. 23 illustrate a series of line segments representingexamples of input function from which the D-A coefficients areascertained for use in connection with the present invention;

FIG. 24 is a block diagram of a simplified D-A convolver circuit afterelimination of the “zero” terms and tailored for the FCS of FIG. 12 andFIG. 20;

FIG. 25 diagrams the D-A circuit of FIG. 24 implementing theaccumulators therein with the accumulator design of FIG. 31;

FIG. 26 diagrams the D-A circuit of FIG. 24 implementing theaccumulators therein with the accumulator design of FIG. 31 in a manneralternative to that shown in FIG. 25;

FIG. 27 is a table illustrating a process for performing thecomputations associated with the derivation of the D-A coefficients forthe FCS of FIG. 12;

FIG. 28 is a table illustrating the results of using the D-A circuit ofFIG. 14;

FIG. 29 diagrams a software-programmable, shift-register implementationof a D-A convolving method;

FIG. 30 diagrams a D-A convolver implemented in a RAM-based circuit; and

FIG. 31 is a circuit diagram illustrating an implementation of one ofthe accumulators shown in FIG. 17;

FIG. 32 is a block diagram of the electronics of the automatic targetrecognition system of the LADAR system of FIG. 1;

FIG. 33 illustrates the handling of three-dimensional data acquired bythe pulse capture electronics, best shown in FIG. C, of the LADAR systemof FIG. 1;

FIG. 34 conceptually illustrates the operation of the LADAR transceiverof FIG. 1 in one particular embodiment of the present invention;

FIG. 35 is a timing diagram of signals employed in one particularembodiment of the present invention;

FIG. 36 illustrates one particular embodiment of a method forautomatically controlling the gain of a receiver in an optical system inaccordance with one aspect of the present invention; and

FIG. 37 illustrates another particular embodiment of a method forautomatically controlling the gain of a receiver in an optical system inaccordance with one aspect of the present invention.

While the invention is susceptible to various modifications andalternative forms, the drawings illustrate specific embodiments hereindescribed in detail by way of example. It should be understood, however,that the description herein of specific embodiments is not intended tolimit the invention to the particular forms disclosed, but on thecontrary, the intention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the invention asdefined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a developmenteffort, even if complex and time-consuming, would be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

The present invention uses a median-value approach to the gain-controlmechanism of an optical system. Statistically, using the median is morerobust than averaging or single-sample techniques. Other methods ofdetermining the median of a large set values are very computationallyintensive that the present invention greatly reduces. Only onecomparison computation is required per input sample. Each one of asequential set of input samples is compared (once) against the medianset-point and a counter increments the number of “greater than” results.After a predetermined number of comparison cycles, the counter valuerepresents how close the gain-control mechanism has achieved the desiredmedian. If the counter is at half maximum value, the median has beenachieved. Any deviation from this “half value” is proportional to theerror. The gain-control mechanism then uses this deviation to “close theloop” and force the error to approach zero over the course of severalcomparison cycles. This process can provide smooth adaptive control asboth external and internal conditions change.

The maximum gain is also typically, though not in all embodiments,limited by an active noise measurement technique. This technique keepsthe Constant False Alarm Rate (“CFAR”) due to detector and backgroundrandom noise from exceeding a predetermined level. The technique isadaptive to changing conditions and allows the maximum gain possiblewhen signals are weak. Statistically, random noise (per time increment)is equally likely during any period of measurement. Thus the level ofrandom noise can be determined during a period when no return signal isexpected. The number of noise occurrences in several periods areaccumulated and then evaluated. If and when that number equals orexceeds an allowable CFAR limit, the gain is prohibited from increasingand may be slightly decreased.

FIG. 1 illustrates one particular embodiment of the present invention, aLADAR system 100. The LADAR system 100 comprises a LADAR transceiver110, some pulse capture electronics (“PCE”) 115, and an automatic targetrecognition (“ATR”) system 120. The LADAR transceiver 110 emits a trainof laser light pulses 125 into a field of view 135 when triggered by asignal from the PCE 115. The field of view 135, more particularly shownin FIG. 2, comprises the volume defined by the propagation boundaries222 of the emitted pulses 125 and the plane 224 in accordance withconventional practice. The plane 224 represents the maximum range of theLADAR transceiver 110. Referring now to both FIG. 1 and FIG. 2, theemitted pulses 125 reflect off a target 130. The return pulses 140 arethen collected by the LADAR transceiver 110 and captured by the PCE 115.The PCE 115 then processes the collected return pulses 140 to generate athree-dimensional data set for processing by the ATR system 120.

FIG. 3 depicts selected portions of the PCE 115 in FIG. 1 in a blockdiagram. The PCE 115 includes a detector array 310, a threshold unit320, a capture and process unit 330, and an automatic gain control(“AGC”) 340. The detector array 310 receives an optical signal 350 fromthe LADAR transceiver 110 and converts it to an analog electrical signal360. The threshold unit 320 produces a continuous digital representation370 of the analog electrical signal 360, from which the capture andprocess unit 330 produces a three-dimensional data set 380. As will bediscussed further below, the capture and process unit 330 deliversinformation 390 to the AGC 340, which delivers a detector array gainsignal 395 to the detector array 310. In accordance with one aspect ofthe present invention, the PCE 115 employs the AGC 340 to variablycontrol the gain of the detector array 310. The gain for the detectorarray 310 is commensurate with the intensity of the return pulse 140. Ifthe intensity of the return pulse 140 is high, then the gain of thedetector array 340 is low and, if the intensity is low, the gain ishigh.

In the illustrated embodiment, the LADAR transceiver 110 continuallysends input data in the form of an optical signal 350 to the PCE 115.This input data is sometimes core data, e.g., the optical signal 350 isthe collected return pulse 140 (shown in FIG. 1). Sometimes, the inputdata is incidental data, e.g., the optical signal 350 is opticalbackground noise from the field of view 135 (also shown in FIG. 1). Notethat noise can also result from the operation of the detector array 310,since electro-optical detectors are notoriously noisy components.Sometimes the input data contains no useful information.

To help separate these conditions, and for other reasons, the PCE 115employs a timing mechanism illustrated in FIG. 4, wherein the captureand process unit 330 acts as a “gate” to control when the optical signal350 is processed by the PCE 115. The LADAR transceiver 110 fires thelaser at a time t_(f1) on the clock 410, which is delivered to the LADARtransceiver 110 by the PCE 115. At some predetermined time t_(re) afterthe time t_(f1), shown on the clock 420, the range gate (not shown) ofthe capture and process unit 330 is enabled for a predetermined periodof time T₁. Any data in the optical signal 350 during this time periodT₁ is assumed to be core data, i.e., a collected, or “true,” returnpulse 140. At a time t_(rd) on the clock 430, the range gate is disabledand the time period T₁ terminates on the assumption that no moredesirable data will be received. At some predetermined time t_(ne) aftert_(rd) ends the time period T₁, but before the laser fires again att_(f2), the noise gate (also not shown) is enabled for a time period T₂that ends before t_(f2). Any data received in the optical signal 350during this period T₂ is assumed to be incidental data, e.g., noise.

The times and widths of the time periods T₁, T₂ are determined by theoperational parameters of the LADAR system 100. The location and widthof the time period T₁ is defined by some expectation of the flight timefor the emitted pulse 125 to reflect back to the LADAR transceiver 110.For instance, one might assume that only targets greater than 100 m butless than 2 km of the LADAR transceiver 110 will be detected andidentified. This assumption, given the speed of light, will yield thetimes t_(re) and t_(rd). The width of the time period T₂ may be selectedon some arbitrary basis, but a width equal to the width of the timeperiod T₁ is employed in the illustrated embodiment. The time period T₃between the time periods T₁, T₂ also may be arbitrarily selected,provided that the sum of T₁, T₂, and T₃ is less than the period of thelaser firing clock 410. However, in the illustrated embodiment, the timeperiod T₃ is set at least as long as it takes the capture and processunit 330 to process any data received in the time period T₁. Also, thetime period T₂ preferably ends after the capture and process unit 330operates on the incidental data as is described further below.

Turning now to FIG. 5A, both the core data acquired during the timeperiod T₁ and the incidental data acquired during the time period T₂ areused by the AGC 340. The AGC 340, in the illustrated embodiment,includes an up/down counter 510 driven by an intensity mediancomputation, at 520, and a constant false alarm rate (“CFAR”)computation, at 530. Note that some alternative embodiments may omit theCFAR computation 530. The intensity median computation 520 drives theup/down counter 510 up and down depending on whether the intensity ofthe return pulse 140 is higher or lower, respectively, than somepredetermined value. Typically, most implementations will employ aplurality of return pulses 140 in this fashion. The CFAR computation 530clamps, or imposes, an upper limit on the content of the up/down counter510, and thus the gain of the detector array 310 (shown in FIG. 3), upondetecting a noise event. In one particular embodiment, the CFARcomputation 530 not only clamps the value, it drives the value down onecount to back off the gain that produced the noise event.

More particularly, when the range gate (not shown) is enabled during theperiod T1, shown in FIG. 4, the intensity median computation 520compares the median intensity of the return pulse 140 against somesetpoint having a predetermined value. As is shown in FIG. 5B, theintensity median computation 520 includes a comparator 560 that receivesas input the pulse intensity and the predetermined setpoint. The resultof the comparison by the comparator 560 drives the up/down counter 565,whose content is proportional to the error in the detector array gain395. The output of the intensity median computation 520 then drives theup/down counter 510 up and down responsive to the result of thatcomparison. In one embodiment, the up/down counter 565 is changed byplus or minus one count depending on the sign of the error. In otherembodiments, the up/down counter 565 can be changed by multiple countsas a function of the sign and magnitude of the error. After apredetermined number of pulses, the content of the up/down counter 565is examined. If the median has been achieved, the intensity of half thepulses will have been above the setpoint and half below. In thiscircumstance, the content of the up/down counter 565 should beunchanged. If the median of the return pulse 140 is higher than thepredetermined value, the intensity median computation 520 drives theup/down counter 510 down, but drives the up/down counter 510 up if theintensity is lower than the predetermined value.

When the noise gate (not shown) is enabled during the period T₂, shownin FIG. 4, the CFAR computation 530 determines how many false alarmsthere are, i.e., noise events detected as returned pulses, and limitsthe value of the up/down counter 510. Note that such noise events may befrom any source, e.g., background noise collected by the LADARtransceiver 110 or noise introduced by the detector array 310.

The content of the up/down counter 510 is converted to an analog signalby the A/D converter 540. The analog count is then used by theattenuator 550 to attenuate the 500 Vdc power supplied to the detectorarray 310 to produce the detector array gain signal 395. The attenuator550 is implemented in one particular embodiment using field effecttransistor (“FET”) technology, but any suitable technology may beemployed.

To further an understanding of the invention, one particularimplementation of the LADAR system 100 shall now be discussed. In thisdiscussion, for this particular implementation:

-   -   FIG. 6 illustrates the LADAR transceiver 110;    -   FIG. 7 illustrates the PCE 115, with FIG. 8 depicting the        capture unit 720, FIG. 9 and FIG. 10 illustrating certain timing        aspects of the PCE 115, and FIG. 11 depicting the process unit        740, and FIG. 12 to FIG. 31 depict various implementations of        the process unit 740; and    -   FIG. 32 and FIG. 33 illustrate the ATR system 120.        Each of the LADAR transceiver 110, PCE 115, and ATR system 120        in this particular implementation shall now be discussed in        turn.

FIG. 6 illustrates the LADAR transceiver 110 of FIG. 1. The LADARtransceiver 110 of the illustrated embodiment is disclosed in:

-   -   U.S. Pat. No. 5,200,606, entitled “Laser Radar Scanning System,”        issued Apr. 6, 1993, to LTV Missiles and Electronics Group as        the assignee of the inventors Nicholas J. Krasutsky, et al.    -   U.S. Pat. No. 5,224,109, entitled “Laser Radar Transceiver,”        issued Jun. 29, 1993, to LTV Missiles and Electronics Group as        the assignee of the inventors Nicholas J. Krasutsky, et al.; and    -   U.S. Pat. No. 5,285,461, entitled “Improved Laser Radar        Transceiver,” issued to Feb. 8, 1994, to Loral Vought Systems        Corporation as assignee of the inventors Nicholas J. Krasutsky,        et al.;        with some differences in operational parameters noted herein.        However, alternative embodiments may employ any suitable LADAR        transceiver known to the art. Indeed, some alternative        embodiments may employ optical receivers without any optical        transmission capability.

More particularly, FIG. 6 provides an exploded view of selected portionsof the LADAR transceiver 110. A gallium aluminum arsenide laser (GaAlAs)612 pumps a solid state laser 614, which emits the laser light energyemployed for illuminating the target 130, shown in FIG. 1 and in FIG. 2.The pumping laser 612 produces a continuous signal of wavelengthssuitable for pumping the solid state laser 614, e.g., in the crystalabsorption bandwidth. The pumping laser 612 has an output power,suitable in the 10-20 watt range, sufficient to actuate the solid statelaser 614. The solid state laser 614 is suitably a Neodymium dopedyttrium aluminum garnet (YAG), a yttrium lithium fluoride (YLF), or aYttrium Vanadate (YVO₄) laser operable to produce pulses with widths of10 to 20 nanoseconds, peak power levels of approximately 10 kilowatts,at repetition rates of 10-120 Khz. The equivalent average power is inthe range of 1 to 4 watts. The preferred range of wavelengths of theoutput radiation is in the near infrared range, e.g., 1.047 or 1.064microns.

Output signals (not shown) from the pumping laser 612 are transmittedthrough an input lens 611 and through a fiber optic bundle 616 which hassufficient flexibility to permit scanning movement of the seeker head110 during operation. The output beam 618 generated by solid state laser614, in the present embodiment, is successively reflected from a firstand a second turning, or folding, mirror 620 and 622 to a beam expander624. The beam expander 624 comprises a series of (negative and positive)lenses adapted to expand the diameter of the beam 618 to provide anexpanded beam 625, suitably by an 8:1 ratio, while decreasing thedivergence of the beam 618 to the expanded beam 625.

The expanded beam 625 is next passed through a beam segmenter 626 fordividing the expanded beam 625 into a plurality of beam segments 627,conceptually represented by a single beam segment 627. The beam segments627 are arrayed on a common plane, initially overlapping, and divergingin a fan shaped array. The beam segmenter 626 preferably segments thebeam into 8 separate but overlapping beam segments 627. The divergenceof the segmented beams 627 is not so great as to produce separation ofthe beams within the optical system, but preferably is sufficientlygreat to provide a small degree of separation at the target, as thefan-shaped beam array is scanned back and forth over the target.

The segmented beams 627 are then reflected from a third turning mirror628, passed through a central aperture 630 of an apertured mirror 632,and subsequently reflected from a scanning mirror 634. The segmentedbeams 627 are reflected from the scanning mirror 634 in a forwarddirection relative to the LADAR system 100, represented by the arrow635. The scanning mirror 634 is pivotally driven by a scanning drivemotor 636 operable to cyclically scan the array of beam segments 627 forscanning the field of view 135, shown in FIG. 1 and in FIG. 2. The arrayof beam segments 627 is preferably scanned across the field of view at arate of approximately 100 Hz. The turning axis of the scanning drivemotor 636 is aligned in parallel with the segmenter wedges of the beamsegmenter 626 whereby the resultant array of beam segments 627 isscanned perpendicularly to the plane in which the beam segments 627 arealigned.

An a focal, Cassigrainian telescope 638 further expands and directs theemitted beam. The telescope 638 includes a forwardly facing primarymirror 640 and a rearwardly facing secondary mirror 642. A protectiveouter dome 644 of a suitable transparent plastic or glass is mountedforward of the secondary mirror 642. A lens structure 646 is mounted incoaxial alignment between the primary mirror 640 and the scanning mirror634, and an aperture 648 is formed centrally through the primary mirror640 in alignment with the lens structure 646.

The transmitted beam segments 627 reflected from the scanning mirror 634are directed through the lens structure 46 for beam shaping, directedthrough the aperture 648 formed centrally through the primary mirror640, reflected from the secondary mirror 42 spaced forwardly of theprimary mirror 640, and then reflected off the primary mirror 640 andout through the transparent dome 644. The resultant emitted pulse 125 isa fan shaped array scanned about an axis parallel to its plane. The beamsegments 627 of the emitted pulse 125 are in side-by-side orientationmutually spaced by a center-to-center distance of twice their diameters.

The emitted pulse 125 is reflected as described above relative to FIG. 1and FIG. 2 back to the LADAR transceiver 110, which receives the returnpulse 140. Returning to FIG. 6, the return pulse 140 is received by thetelescope 638, and reflected successively by the primary mirror 640 andthe secondary mirror 642, the lens assembly 646, and the scanning mirror634, toward the apertured mirror 632. Because the return pulse 140 is ofsubstantially larger cross-sectional area than the emitted pulse 125, itstrikes substantially the entire reflecting surface of the aperturedmirror 632, and substantially all of its energy is thus reflectedlaterally by the apertured mirror 632 toward collection optics 650. Thecollection optics 650 include a narrow band filter 652, for filteringout wave lengths of light above and below a desired laser wavelength toreduce background interference from ambient light. Note that the returnpulse 140 comprises multiple beam segments 627 just as does the emittedpulse 125.

The collected return pulse 140 then passes through condensing optics 654for focusing the collected return pulse 140, and then a fourth turningmirror 656 re-directs the collected return pulse 140 toward a focusinglens structure 658 adapted to focus the collected return pulse 140 uponthe receiving ends 660 of a light collection fiber optic bundle 662. Theopposite ends of each optical fiber 662 are connected to illuminatediodes 664 in the detector array 310. Note that the detector array 310actually comprises a portion of the PCE 115, but is presented in thisdiscussion of the LADAR transceiver 110 for clarity.

The detector array 310 converts the laser light signals of the collectedreturn pulse 140 to electrical signals that are conducted to theremainder of the PCE 115, first shown in FIG. 1, as is discussed morefully below. The fiber optic bundle 662 includes nine fibers, eight ofwhich are used for respectively receiving laser light corresponding torespective transmitted beam segments 627 and one of which viewsscattered light from the transmitted pulse to provide a timing startpulse. The light received by the ninth fiber is transmitted to one ofthe diodes 664 of the detector array. The input ends 660 of the fibers662 are mounted in linear alignment along an axis which is perpendicularto the optical axis. The respective voltage outputs of the detectors 664thus correspond to the intensity of the laser radiation reflected frommutually parallel linear segments of the field of view 135 shown in FIG.1 and FIG. 2, which is parallel to the direction of scan.

One particular implementation of the LADAR transceiver 110 splits asingle 0.2 mRad 1/e2 laser pulse into septets with a laser beamdivergence for each spot of 0.2 mRad with beam separations of 0.4 mRad.The fiber optical array 310, shown in FIG. 6, includes the fibers 662,which have an acceptance angle of 0.3 mRad and a spacing between thefibers 662 that matches the 0.4 mRad far field beam separation. The beamsegments 627 are vertically spread by 0.4 mRad as it produces thevertical scan angle.

Note that alternative embodiments may employ LADAR transceivers otherthan the LADAR transceiver 110 discussed above. Other suitabletransceivers include those disclosed in, inter alia, the followingpatents:

-   -   U.S. Pat. No. 4,515,471, entitled “Scanning Laser Radar,” issued        May 7, 1985, to LTV Aerospace and Defense Company as the        assignee of the inventor Dayton D. Eden;    -   U.S. Pat. No. 4,515,472, entitled “Agile Receiver for a Scanning        Laser Radar,” issued May 7, 1985, to LTV Aerospace and Defense        Company as the assignee of the inventor Albert B. Welch;    -   U.S. Pat. No. 4,528,525, entitled “Scanning Laser for a Scanning        Laser Radar,” issued Jul. 9, 1985, to LTV Aerospace and Defense        Company as the assignee of the inventors Dayton D. Eden et al.;        Alternative embodiments might employ other techniques to provide        additional capabilities, such as that disclosed in:    -   U.S. Pat. No. 6,262,800, entitled “Dual mode semi-active        laser/laser radar seeker,” issued Jul. 17, 2001, to Lockheed        Martin Corporation as the assignee of the inventor Lewis G.        Minor;        Some alternative embodiments may also incorporate additional        techniques to improve the quality or resolution of the data that        can be obtained from the return pulse 140. Such techniques are        disclosed in, inter alia, the following patents:    -   U.S. Pat. No. 5,701,326, entitled “Laser Scanning System With        Optical Transmit/Reflect Mirror Having Reduced Received Signal        Loss,” issued Dec. 23, 1997, to Loral Vought Systems Corporation        as the assignee of the inventor Edward Max Flowers;    -   U.S. Pat. No. 5,285,461, entitled “Method for Increasing        Single-Pulse Range Resolution,” issued Feb. 8, 1994, to Loral        Vought Systems Corporation as the assignee of the inventors        Nicholas J. Krasutsky, et al.    -   U.S. Pat. No. 5,898,483, entitled “Method for Increasing LADAR        Resolution,” issued Apr. 27, 1999, to Lockheed Martin        Corporation as the assignee of the inventor Edward Max Flowers;    -   U.S. Pat. No. 6,115,113, entitled “Method for Increasing        Single-Pulse Range Resolution,” issued Sep. 5, 2000, to Lockheed        Martin Corporation as the assignee of the inventors Stewart W.        Flockencier.        Thus, practically any LADAR transceiver known to the art may be        employed to implement the present invention. Some embodiments        may also employ additional techniques for acquiring other kinds        of data to enhance application of the three-dimensional data set        380 to its end use.

FIG. 7 conceptually illustrates, in a block diagram, one particularembodiment 700 of the PCE 115 of the LADAR system 100 of FIG. 1. The PCE115 initiate the firing of the laser transmitter in the LADARtransceiver 110 and determine the time-of-flight (range) and intensityof the return pulses collected by the LADAR transceiver 110. A matchedfilter technique extracts the range and intensity information from thedata stream generated from the collected return pulse 140. This processproduces accurate data under low signal-to-noise conditions and withwidely varying reflectivity returns, and can isolate secondary returnsfrom interference such as from under trees or behind camouflage nets.The PCE 115 output the results of the analysis, a three-dimensional dataset 380, to the ATR system 120 for execution of the targetingalgorithms.

More particularly, from the standpoint of the invention, the collectedreturn pulse 670 is digitized and stored and a detailed analysis thatutilizes many time samples is performed. This analysis “slides” atemplate across the stored sequence of samples to find the best match.The location of this best match is proportional to the range of thecaptured pulse while the strength of the match is related to itsintensity. The template averages or convolves many samples together toimprove the signal to noise ratio and to find the center of the returnpulse. Using this method, signal amplitude variations may be ignored. Anappropriate template can also be used for second-pulse logic to extractsecondary returns even if the later pulse is partially overlapped by astronger primary return. This analysis yields a data set representingthe shape of the returned pulse 140. This shape is compared to that ofthe emitted pulse 120 to determine flight time of the pulses 120, 140 toand from the target. This method avoids certain limitations experiencedin prior-art, analog edge detection systems relating to power, range, orfrequency considerations.

Returning to FIG. 7, the PCE 115 receives the optical signal 350, firstshown in FIG. 3, and the laser clock 710 (i.e., the clock that controlsthe timing of the laser pulse emission) from the LADAR transceiver 110.The discussion relative to FIG. 6 set forth above assumes that the LADARtransceiver 110 is collecting returned pulses 140 upon the reflection ofa transmitted pulse 125. However, as is apparent from the discussionrelative to FIG. 3 above, the LADAR transceiver 110 collects informationtransmitted to the detector array 310 other than returned pulses 140.Indeed, the LADAR transceiver continuously collected information that isoutput to the PCE 115 as the optical signal 350.

Still referring to FIG. 7, the PCE 115 processes the received opticalsignal 350 and outputs a three-dimensional data set 380. The PCE 115, inaccordance with various aspects of the present invention, include thedetector array 310, a threshold unit 320, a capture unit 720, a randomaccess memory (“RAM”) 730, a process unit 740, an automatic gain control(“AGC”) 340, and a multiphase clock generator 760. In the course ofprocessing the optical signal 350, the PCE 115 controls the gain of thedetector array 310 through the AGC 340 as was discussed relative to FIG.5A above.

As previously mentioned, the detector array 310 comprises a plurality ofphotodiodes 664. In the illustrated embodiment, the photodiodes 664 areavalanche photodiodes (“APD”). However, as will be appreciated by thosein the art having the benefit of this disclosure, a wide variety ofdetector technologies may instead be employed to implement the detectorarray 310. Any such suitable detector technology may be employed, andthe invention is not limited by this aspect of the implementation. Theoptical signals collected by the photodiodes 664 are then amplified bythe post-amplifiers 765 and output to the threshold unit 320.

The threshold unit 320 converts the analog, collected return pulse 140signal to a digital signal using a 1 GHz flash converter process. A bankof voltage comparators (not shown) is employed, and comparison isaccomplished by first converting each pulse signal to a string ofdigital signals. Each portion of the collected return pulse 140 equalsthe instantaneous amplitude of the signal received at that moment. Inthe illustrated embodiment, the threshold unit 320 employs a comparisoncircuit (not shown) comprising seven individual, nonlinear comparators(not shown). The comparators are preferably flash converters gangedtogether and spaced apart in their threshold in a logarithmic function,between 30 mV and 1 V, based on expected return. The collected returnpulse 140 is fed to all seven comparators at once, but each has adifferent reference voltage 766, and levels being spaced in logarithmicintervals over the expected voltage range. Starting at the lower level,all the analog comparators with digital outputs operate like an operableamplifier with no feedback. At every clock, the highest comparator turnson. The threshold unit 320 outputs the resultant continuous digitalrepresentation 370 of the optical signal 350.

Note that more than seven comparators can be used to increase theamplitude resolution and reduce the effects of sampling jitter andnoise. Note further that the values of the reference voltages 766 forthe multi-level comparison by the threshold unit 320 are set bypredetermined values stored in the register 768. However, this is onlyone approach to setting these values. For instance, the values could bestored in the control registers 770, or even the RAM 730. The controlregisters 770 are used to store operational parametrics. These valuesmay be loaded prior to use of the LADAR system 100 through the test port772, but, again, alternative techniques may be employed.

The capture and process unit 330 includes the capture unit 720, RAM 730,and process unit 740 capture the collected return pulse 140 and processit. The capture unit 720 samples the digitized representation of thecollected return pulse 740 output by the threshold unit 320 at 500 MHz.The capture unit 720 converts the sampled signal 370 into a 3-bit word(or, “thermometer code”) proportional to the peak of the collectedreturn pulse 140, and stores the coded samples in the RAM 730. Theprocess unit 740 then performs the detailed analysis mentioned above onthe data samples stored in the RAM 730. The result of this analysis isoutput to the input/output (“I/O”) unit 774, which conditions theresults to produce and output the three-dimensional data 380.

The capture unit 720 comprises a gate array and is better shown in FIG.8. As shown in FIG. 8, the threshold unit 320 feeds the digital signal370 to the capture unit 720. The capture unit 720 has three functions:to sample the input waveform at 500 MHz, convert the sample into the3-bit thermometer code proportional to the peak of the input signal inan encoder 810, and to time de-multiplex the signal to 125 MHzcompatible with the RAM 730.

A 4:1 time demultiplexer 820 is provided which, in operation, allowseight nanoseconds for the encoding of a signal. From the capture unit720, four three-bit samples are then stored in the RAM 730. Thus, thecapture unit 720 samples the status or state of the bank of comparatorsin the threshold unit 320 every two nanoseconds to determine how many ofthe comparators are turned on, assigns a digital word (0, 1, 2 through7) indicating that determination, and stores the digital word in the RAM730.

In the illustrated embodiment, the capture unit 720, as well as someother aspects of the PCE 115, is implemented in a field programmablegate array (“FPGA”) 780. Current FPGA technology is not capable ofoperating at 1 GHz frequencies, unlike the emitter-coupled logic (“ECL”)of the prior art. Thus, the illustrated embodiment employs a multiphaseclock generator 760 that generates a multiphase clock signal from aninput clock signal, each phase of which is used to by the capture unit720 to time a respective sampling. The illustrated embodiment uses thismultiphase clock technique to sample digitized, collected return pulse140 at a rate effectively higher than the basic FPGA clock rate of 125MHz.

Consider, for instance, the particular implementation shown in FIG. 9and FIG. 10. In this particular implementation, the FGPA 780 in whichthe capture unit 720 is implemented operates on a nominal 125 MHz clock910. The 125 MHz clock signal 915 from the 125 MHz clock 910 is input tothe multiphase clock generator 760 through a delay line 920. The delayline 920 imposes a 2 ns delay. The multiphase clock generator 760 thengenerates the four-phase clock signal 930, each phase separated by 2 ns.Each phase 1010 of the four-phase clock signal 930 drives a samplingcycle such that the capture unit 720 is sampling at a 500 MHz rate. Thecapture unit 720 is sampling as though driven by four time-multiplexed125 MHz clocks. The samples are stored in the RAM 730 when acquired, andthus are also stored in a time-multiplexed fashion. Consequently, thestored samples in the RAM 730, when read out by the process unit 740,appear as if they had been sampled at 500 MHz by a single, 500 MHz clocksignal. Similarly, a 125 MHz eight-phase system would achieve aneffective 1 GHz sampling rate. In addition, an increase in chipperformance would allow an increase in effective sampling rate orreduction in the number of phases.

Returning to FIG. 7, the process unit 740 then reads the samples fromthe RAM 730 and processes them into the three-dimensional data set 380using the matched filter technique mentioned above. The process unit 740comprises, as is shown in FIG. 11, a convolution circuit 1110 and a peakdetect circuit 1120. The convolution circuit 1110 analyzes the samplesusing a matched filter and delivers the result of the analysis to thepeak detect circuit 1120. The peak detector circuit 1120 identifies thepeak of the convolved results, which correlates to the arrival time ofthe return pulse 140. This process may extract higher resolution resultsby use of the D-A procedure described below. The peak detector circuit1120 delivers the range data 1130 and intensity data 1140 for the returnpulse 140, which comprise the three-dimensional data set 380.

The convolution circuit 1110 includes a matched filter 1112 and, in someembodiments, the optional linearize circuit 1114. The linearize circuit1114 strips out the non-linearity introduced by the non-linear thresholdunit 320. The matched filter 1112 may be any FIR filter known to theart. The matched filter 1112 is loaded with a set of coefficientsrepresentative of the expected shape of the return pulse 140. This setof coefficients defines the template discussed above. The convolutioncircuit 1110 produces evaluation numbers indicative of the degree ofcorrelation between the template and the return pulse 140.

The design of convolution circuits, e.g., the convolution circuit 1110,is well-known in the art. Conventional convolution circuits feed thesamples serially into the filter thereof, each sample being fed one at atime. In this conventional approach, the template is shifted across thestored sequence in discrete steps defined by the sampling rate at whichthe analog return pulse is digitized. Each discrete step is no wider ornarrower than an individual sample and the resolution of any measurementobtained therefrom is determined by the sampling rate in theanalog-to-digital conversion.

In one particular embodiment, the convolution circuit 1110 effectivelyimproves the sampling rate at which the return pulse is digitized. Theconvolution circuit 1110 does so by “interpolating” between samples. Foran R:1 interpolation, where R>1, each sample is clocked out of thebuffer 222 and into the convolution circuit 1110 R times before clockingout the next sample so that each sample is fed into the convolution Rtimes.

Thus, the convolution circuit 1110 repeatedly convolves each one of atleast a portion of the buffered samples R times, wherein R>1. Thisconvolution technique may also be conceptualized as shifting thetemplate in steps smaller than the input samples such that a correlationdetermination can be made at points in between the discrete samples.This convolution technique therefore produces an effective sampling ratepotentially much higher than the actual sampling rate without directlyimpacting the data acquisition. Consequently, the resolution of therange extracted therefrom is improved by a factor of R.

The peak detect circuit 1120 processes the evaluation numbers from theconvolution circuit 1110 to determine the point in time where a “bestmatch” occurs between the return pulse and the template. “Best match” isdefined, in this context, as the set of samples among those filteredwhose correlation with the template is highest. By identifying theportion of the sampled return signal that best matches the storedtemplate, the peak detect circuit 1120 effectively determines the timeat which the return pulse 140 was received.

Thus, the peak detect circuit 1120 detects the center of energy for thereturn pulse 140 from the convolution results output by the convolutioncircuit 1110. Technically, as those in the art having the benefit ofthis disclosure will appreciate, the peak detect circuit 1120 detectsthe peak of the processed, digitized samples, which theoreticallyrepresents the peak of the return signal 1120. However, in practice,this is not always the case as the data may be corrupted or polluted.Thus, the peak of the processed samples indicates the return pulse'scenter of energy, from which the time at which the pulse was receivedmay be more accurately determined.

As noted above, any convolution circuit including a matched filter knownto the art can be employed to perform the convolution of the sampleswith the template, provided it is modified to clock each sample R timesto perform an R:1 interpolation. That is, any convolution circuit knownto the art may be used to implement the convolution circuit 1110.However, in the interest of completeness, two implementations for theconvolution circuit 1110 are disclosed. FIG. 12 to FIG. 16 illustrate afirst implementation and a second convolution circuit employing aprocessing technique reducing the amount of hardware needed to implementit is illustrated in FIG. 17 to FIG. 31. Both of these convolutioncircuits are also disclosed in:

-   -   U.S. Pat. No. 6,115,1113, entitled “Method for Increasing        Single-Pulse Range Resolution,” issued Sep. 5, 2000, to Lockheed        Martin Corporation as the assignee of the inventor Stuart W.        Flockencier (“the '113 patent”), which describes an        over-sampling method for increasing range resolution; and    -   U.S. Pat. No. 5,511,015, entitled “Double-Accumulator        Implementation of the Convolution Function,” issued Apr. 23,        1996, to Loral Vought Systems Corporation as the assignee of the        inventor Stuart W. Flockencier, which describes a method for        reducing FIR complexity and whose teachings were adapted to the        over-sampling method in the '113 patent.        Both of these patents are commonly assigned herewith to what is        now Lockheed Martin Corporation.

Turning now to the first implementation of the convolution circuit 1110,FIG. 12 graphs a filter coefficient stream (“FCS”), i.e., thecoefficients defining the template to be matched. The coefficients arerepresented in the figures by the notation A_(t), where the subscript“t” shows the relative position of the coefficient in the FCS in thetime domain. Using Equation 1, set forth below, the convolution of thissample function is illustrated in FIG. 13 for times (t⁻¹, t₀, t₁, . . .t₆) for each of “n” samples.

${C_{x\; y}(m)} = {\frac{1}{N}{\sum\limits_{K = 0}^{N - 1}\;{{x(k)}{y\left( {m - k} \right)}}}}$

FIG. 14 shows a high-speed circuit approach for implementing thisconvolution, using five multipliers 1405 (only two indicated) arrangedin parallel with a summation circuit 1410 providing the convolutionresults, depicted as Z_(n). In practice, the summation circuit 1410 isimplemented by several 2-input adders 1605, shown in FIG. 16 (only twoindicated). Specifically a total of N-1 adders are employed in thisimplementation. The table shown in FIG. 15 illustrates the summationoutputs as each of the samples is processed from the lowest-positionedof the multipliers to the highest-positioned of the multipliers, andassuming that A₀ is the first data sample having a non-zero value.Accordingly, for the convolution implementation shown in FIG. 14 to FIG.16, four adders and five multiply operations are required for eachoutput value of “n”. Note that each sample is read R times, where R>1,before the next sample is read to implement an R:1 interpolation as wasdiscussed above.

In the second implementation illustrated in FIG. 17 to FIG. 31, theconvolution circuit 1110 uses a double-accumulator technique to reducethe number and complexity of multiply-and-add operations typicallynecessary per sample for the approach. The amount of reduction dependsupon the shape of the filter function, i.e., the numerical relationshipbetween adjacent coefficients. If the filter function is expressed as aseries of “piece-wise linear” segments, then this double-accumulatortechnique reduces the number of necessary multiply-and-add operations. Asequence of coefficients are said to be piece-wise linear if eachsuccessive term represents a constant change in value from the previousterm.

Turning now to FIG. 17, this embodiment of the matched filter includesfirst and second accumulators 1701, 1702 and a plurality of multipliers1705-1712. Each of the multipliers 1705-1712 has an input and an outputand each is configured to multiply a respective input data sample1713-1720 by a derived D-A (double-accumulator) coefficient. The firstaccumulator 1701 receives the outputs of the multipliers 1705-1712 froma summer 1723. The second accumulator 1702 receives an output 1703 fromthe first accumulator 1701. In a more specific implementation, thematched filter further includes a shift register (not shown) forsequentially receiving input data samples 1713-1720 and for providingthe input data samples 1713-1720 to the inputs of the multipliers.

Preferably, a double-accumulator is implemented using a set of D-Acoefficients derived from the input filter coefficient stream (“FCS”),e.g., the FCS in FIG. 12. In most implementations, the D-A coefficientsare obtained by taking the second derivative of the FCS, discussedfurther below relative to FIG. 17, FIG. 19, and FIG. 20. The secondderivative operation, on the piece-wise linear coefficient stream,forces most of the D-A coefficients to zero. The remaining non-zeroterms are “small” numbers compared to the original coefficients. Onceascertained, the non-zero D-A coefficients are used to weight the inputdata samples.

The weighting technique can be accomplished by multiplying each D-Acoefficient by a separate input sample, as will be discussed relative toFIG. 17. Note that each sample is read R times, where R>1, before thenext sample is read to implement an R:1 interpolation as was discussedabove. The products are then summed together along with the result of aprevious multiplication of the same D-A coefficients with differentinput data samples. This first sum is added to another number to form asecond sum. The other number is the previous value of the second sum.The second sum is the result. The resultant output data stream isexactly the same as the method would compute.

Such a double-accumulator can be implemented in a variety of circuits.For example, a register-based double accumulator may be implementedusing a shift register to sequentially move input data across the inputsto a plurality of multipliers. The multipliers are used to multiply theinput data by selected D-A coefficients. The products are summedtogether and provided to a first accumulator. The first accumulatorprovides its output to the input of a second accumulator. The secondaccumulator provides the result.

This particular double accumulator implementation is, as discussedabove, deployed in a RAM-based convolver. The RAM 730, shown in FIG. 7and in FIG. 11, holds the data samples and D-A coefficients. An addressgenerator (not shown) directs the RAM 730 to provide appropriate pairsof input data samples and D-A coefficients. These pairs are provided toa multiply-accumulator. The output of the multiply-accumulator iscoupled to a accumulator. The output of the accumulator is the result.Note that other implementations may employ other types of memory thatmay buffer the data samples.

More particularly, a D-A convolver circuit is constructed fromcoefficients derived from the filter coefficient stream (“FCS”). Thesecoefficients are referred to as “D-A coefficients.” Referring to FIG.12, an example of a FCS is shown plotted on a graph as the numericalseries 1,2,3,2,1 formed from two piece-wise linear segments. This shortseries is chosen to keep the example simple and should not be viewed asa limit on the D-A method. Note that a similar sequence 1,2,3,4,5, . . ., 5,4,3,2,1 is also formed from two piece-wise linear segments and willyield the same number of non-zero D-A coefficients (three) regardless ofthe number of original terms. The approach would contain onemultiplier-summer per each original term, not three as in the D-Amethod.

The D-A coefficients are preferably ascertained from the secondderivative of the FCS. FIG. 17 graphs the first derivative of the FCS inFIG. 12. Generally, the first derivative is found by Equation 2:X′ _(n)=(

x _(n))/(

t)=x _(n) −x _(n)+1FIG. 19 graphs the second derivative of the FCS in FIG. 12. Generally,the second derivative can be found by equation (3):X″ _(n)=(

x _(n)′)/(

t′)=x _(n) −x _(n)″+1The resulting values are the D-A coefficients. FIG. 20 graphs theresulting D-A coefficients against the axis B_(n). In the D-A circuitry,the input sample stream will be weighted, preferably multiplied, bythese coefficients. Since most of the coefficients are zeros, the D-Acircuitry requires only a few multipliers.

D-A coefficients are readily ascertained from most any functionexpressed as a series of line segments. One D-A coefficient isdesignated for each point where two line segments meet. The value of aD-A coefficient is equal to the change in slope (second derivative) fromthe first line segment to the second. Examples of this method are shownin FIG. 21 to FIG. 23.

Referring first to FIG. 21, all line segment slopes are indicated by“m=” and all D-A coefficients are circled. Specifically, FIG. 21 showsline segment 2101 with slope m=0 adjoining to line segment 2102 havingslope m=1. Therefore, the D-A coefficient at point 2115 is “1” (1−0).FIG. 21 also shows line segment 2102 with slope m=1 adjoining to linesegment 2103 having slope m=−1. Therefore, the D-A coefficient at point2116 is “−2” ((−1)−1). In addition, FIG. 21 shows line segment 2103 withslope m=−1 adjoining to line segment 2104 having slope m=0. Therefore,the D-A coefficient at point 2117 is “1” (0−(−1)).

FIG. 22 shows line segment 2205 with slope m=0 adjoining to line segment2206 having slope m=2. Therefore, the D-A coefficient at point 2218 is“2” (2−0). FIG. 22 also shows line segment 2206 with slope m=2 adjoiningto line segment 2207 having slope m=0. Therefore, the D-A coefficient atpoint 2219 is “−2” (0−2). In addition, FIG. 22 shows line segment 2207with slope m=0 adjoining to line segment 2208 having slope m=(−0.5).Therefore, the D-A coefficient at point 2220 is “−0.5” ((−0.5)−0). FIG.22 shows line segment 2208 with slope m=(−0.5) adjoining to line segment2209 having slope m=0. Therefore, the D-A coefficient at point 2222 is“0.5” (0−(−0.5)).

FIG. 23 shows line segment 2310 with slope m=0 adjoining to line segment2311 having slope m=2. Therefore, the D-A coefficient at point 2318 is“2” (2−0). FIG. 23 also shows line segment 2311 with slope m=2 adjoiningto line segment 2312 having slope m=(−3). Therefore, the D-A coefficientat point 2323 is “−5” ((−3)−2). In addition, FIG. 23 shows line segment2312 with slope m=(−3) adjoining to line segment 2313 having slope m=0.Therefore, the D-A coefficient at point 2324 is “3” (0−(−3)). Lastly,FIG. 23 shows line segment 2313 with slope m=32 (0) adjoining to linesegment 2314 having slope m=1.5. Therefore, the D-A coefficient at point2325 is “1.5” (1.5−0).

Returning to FIG. 17, the multipliers 1705-1712 multiply input samplevalues by the ascertained D-A coefficient values. Depending upon theFCS, more or less multipliers may be used. The products from themultipliers 1705-1712 are added together by adder 1723. The sum is fedto the first accumulator 1701, which accumulates a running summation ofsequential outputs from the adder 1723 as each sample is processed. Theresult 1703 from the first accumulator 1701 is fed to the secondaccumulator 1702, which accumulates a running summation of sequentialoutputs from the accumulator 1701 as each sample is processed. Theresult from the second accumulator 1702 is the result of the convolution(or the filtered signal value) 1704. Using the example from FIG. 17 andFIG. 18, the circuitry would require only three multipliers because fiveof the D-A coefficients are zeros, as shown in FIG. 20.

FIG. 24 illustrates a more tailored example of the circuit in FIG. 17.The input values (samples) are designated by “A_(n-1)”, or similarnotations. These samples are multiplied by D-A coefficients inmultipliers 2403, 2404, and 2405. The multipliers 2403-2405 feed asummer 2406 which sums the three products. The summer feeds the firstaccumulator 1701. The first accumulator 1701 feeds the secondaccumulator 1702. The second accumulator 1702 yields the result of theconvolution (or a filtered signal). The first accumulator 1701 and thesecond accumulator 1702 operate in the same manner as described inconnection with the accumulator 1701 and 1702 of FIG. 17.

Still referring to FIG. 24, the input values (samples, A_(n-t)) areweighted (multiplied) by the non-zero D-A coefficients from FIG. 20.Specifically, sample A_(n-6) represents the most-recently input datasample and is fed into a multiplier 2403 to be multiplied by the D-Acoefficient “1”, which corresponds to the right most D-A coefficient “1”in FIG. 20. A_(n-3) is a sample taken three samples before A_(n-6).A_(n-3) is fed into multiplier 2404 to be multiplied by the D-Acoefficient “−2”; this “−2” corresponds to the “−2” D-A coefficient fromFIG. 20. A_(n) is a sample taken 6 samples before sample A_(n-6). An isfed into multiplier 2405 to be multiplied by the D-A coefficient “1”this “1” corresponds to the left most D-A coefficient in FIG. 20. Thetwo sample spaces between A_(n) and A_(n-3) correspond to the two “0”spaces between D-A coefficients “1” and “−2” in FIG. 20. Similarly, thetwo sample spaces between A_(n-3) and A_(n-6) correspond to the two “0”spaces between D-A coefficients “−2” and “1” in FIG. 20.

FIG. 31 shows the circuitry comprising an accumulator 3100 (e.g. theaccumulators 1701, 1702 of FIG. 17, FIG. 24). The input to theaccumulator 3100 enters a summer 3105. The output of the summer 3105 isstored in a memory file 3110. The memory file 3110 may comprise nearlyany re-writable memory mechanism such as a DRAM, a register file,flip-flops, latches, etc. The output of the memory file 3110 providesthe second input to the summer 3105 via line 3115 and provides theoutput of the accumulator 3100. FIG. 25 shows the D-A circuit of FIG. 24implementing the accumulators 1701, 1702 with the accumulator 3100 ofFIG. 31. Alternatively, the summers 2401 and 3105 may be consolidated asshown in FIG. 26.

More particularly, FIG. 26 shows a complete D-A circuit including threemultipliers 2403, 2404 and 2405, and a pair of accumulators 2605 and3100 arranged for performing the convolution of the functions discussedand illustrated in connection with FIG. 17 and FIG. 18. A clock line2602 is used to store and release data from the memory file 3110 and toadvance the input data to be convolved. The circuit of FIG. 26 canperform the convolution of FIG. 17 and FIG. 18 with only threemultiplies per convolution result (Z_(n)). The method requires fivemultipliers. For longer convolver streams, the savings are greater.

As an example for a specific DSP application concerning a LASER radar,the clock line 2610 provides a clock at a rate of 83.3 MHz, with a D-Acoefficient designated for the input function every 4 nanoseconds.

While three multipliers 1803, 1804 and 1805 are shown, the multipliers1803 and 1804 are unnecessary and can be bypassed because they arearranged to multiply by one. Moreover, a register performing a simplebinary shift operation can replace the multiplier 1804 because itsfunction, multiplying by a factor of two, is effected by a single binaryshift. Such circuitry reduction is common, expected and considered to beunderstood in the illustrated embodiments herein because, by theirnature, D-A coefficients are generally small numbers.

FIG. 27 and FIG. 28 illustrate the mathematics of these computationsprovided by the convolution result (Z_(n)) of FIG. 26. The data to beconvolved is shown along the top horizontal axis of the table of FIG.27, and the left vertical axis illustrates the convolving coefficientswhich are effectively slid across the data. At each sample time, thedata values are read, multiplied by the D-A coefficients, summed, anddouble accumulated, as previously discussed. Note that the results arethe same as the implementation of FIG. 15. Since most of the D-Acoefficients are equal to zero and two of the coefficients are equal toone, far less multiplies are necessary. In FIG. 28, a table shows theresults of the convolution performed by the D-A circuit of FIG. 26. InFIG. 26 and FIG. 27: “n” is the sample count; “B_(n)” 2603 is the outputof summer 2601 for the sample count given by “n”; “C_(n)” is the outputof the first accumulator 2606 for the sample count given by “n”; and“Z_(n”) is the convolution result at the time of the sample count “n”.

The matched filter 1114, first shown in FIG. 11, can be implementedusing a variety of different circuits or software (including firmware)implementations. FIG. 29 shows a software programmable, shift registerimplementation 2900 of the D-A convolving method. Data samples 2902 areshifted into a shift register 2901. Each stage of the shift register2901 has a respective output 2905 that enters a programmable multiplier2903. The programmable multiplier 2903 selects which outputs 2905 to useand how to weight each of the selected outputs. Using the example ofFIG. 12 and FIG. 20, the multiplier 2903 uses only the first, third andsixth stages of the shift register 2901. Using the D-A coefficients fromFIG. 12, the first stage is weighted by 1, the third stage is weightedby (−2), and the sixth stage is weighted by 1. The products from themultiplier 2903 are fed to a summer 2903 a. The summer 2903 a feeds thefirst accumulator 2904. The first accumulator 2904 feeds the secondaccumulator 2906. And finally, the result is output from the secondaccumulator 2906.

A RAM-based convolver may also be used to implement the matched filter1114. FIG. 30 shows a D-A convolver 3000 implemented in the hardware orsoftware RAM-based matching filter. RAM memory 3004 holds input data3001 and D-A coefficients (1, (−2) and 1, from the example of FIG. 20).Each clock cycle, multiply-accumulator 3003 multiplies a single D-Acoefficient by a single data sample. Address generator 3002 isresponsible for sequencing the correct order of data samples and D-Acoefficients into multiplier-accumulator 3003. The output ofmultiply-accumulator 3003 feeds accumulator 3005. The accumulator 3005is clocked at a fraction of the system clock rate. The fraction is equalto (1/(number of D-A coefficients)). This allows the accumulator 3005 toreceive and sum, over 3 system clock cycles, the same three data pointsthat accumulator 2605 would receive and sum over a single clock cycle.

Returning now to FIG. 7, the timing of the operations in the captureunit 720 and the process unit 740 are controlled, indirectly, by thelaser clock 710. The laser clock 710 is input to a timer 745, whoseoutput is input to the capture unit 720 and a sequencer 750. The outputof the sequencer 750 is input to the process unit 740.

The output of the capture and process unit 330 includes the pulseintensity and pulse detect data input to the AGC 340 as discussed aboverelative to FIG. 5A. This information is acted upon as previouslydiscussed to control the gain of the detector array 310 via the detectorarray gain signal 395, also shown in FIG. 7. The detector array 310includes not only the photodiodes 660, but also the post-amplifiers 765,as was previously discussed. The post amp array 755 amplifies theoptical signal picked up by the photodiodes 660, and thus directlydetermines the gain of detector array 310 as a whole. The detector arraygain signal 395 is applied to the post amp array 755 to control thatgain.

Still referring to FIG. 7, the PCE 115 also includes a heater, includinga sensor, 775 and a heater control 780. The heater 775 and heatercontrol 780 maintain the operational temperature of the detector array310 within some predetermined temperature range. In some applications,heating has been found to improve the efficacy of the detector array310. Embodiments not intended for cooler operational environments may,however, omit the heater 775. Thus, the heater 775 is not necessary tothe practice of the invention.

Turning now to the ATR System 120, FIG. 32 conceptually illustrates theelectronics 3200 of the ATR system 120 in a block diagram. Note that theillustrated embodiment of the LADAR system 100 is a flying submunition,although the invention is not so limited. The end use of the LADARsystem 100, in the illustrated embodiment, is to identify and destroyenemy targets 130 of interest. Thus, the LADAR system 100 includes theATR system 120. However, the invention admits wide variation in end usesand, consequently, may be employed in numerous applications inalternative embodiments. Many of these alternative embodiments may usethe three-dimensional data set 380 in different ways such that they omitor replace the ATR system 120. The ATR system 120, therefore, is notnecessary to the practice of the invention.

Returning to FIG. 32, the ATR system 120 includes a processor 3205communicating with some storage 3210 over a bus system 3215. The storage3210 may include a hard disk and/or RAM and/or removable storage, suchas a floppy magnetic disk or an optical disk. In the illustratedembodiment, the storage 3210 includes at least a read-only memory 3218in which operating system 3220 and the application 3225 are encoded andthe RAM 730 in which the data structure 3230 is encoded. The datastructure 3220 stores the three-dimensional data set 380 acquired asdiscussed above by the PCE 115 from the captured return pulse. Theprocessor 3205 runs under the control of the operating system 3230,which may be practically any operating system known to the art. Theprocessor 3205, under the control of the operating system 3230, invokesthe application 3225 on startup. The application 3225, when executed bythe processor 3205, performs any processing or preprocessing on thethree-dimensional data set 380 stored in the data structure 3225. Moreparticularly, the application 3225 implements, in the illustratedembodiment, the method 3300 shown in FIG. 33 and discussed more fullyimmediately below.

FIG. 33 illustrates the handling by the processor 3205, through theapplication 3265, of the three-dimensional data set 380 encoded on thestorage 3210. The LADAR three-dimensional data set 380 is stored in arow column format (at 3350). This further processing generally involvespreprocessing (at 3352), detection (at 3354), segmentation (at 3356),feature extraction (at 3358), and classification (at 3360). Each ofthese processing steps shall now be discussed further.

Generally, the preprocessing (at 3352) is directed to minimizing noiseeffects, such as identifying so-called intensity dropouts in theconverted three-dimensional data set 380, where the range value of thedata is set to zero. Noise in the converted three-dimensional data set380 introduced by low signal-to-noise ratio (“SNR”) conditions isprocessed so that performance of the overall LADAR system 100 is notdegraded. In this regard, the three-dimensional data set 380 is used sothat absolute range measurement distortion is minimized, edgepreservation is maximized, and preservation of texture step (thatresults from actual structure in objects being imaged) is maximized.

In general, detection (at 3354) identifies specific regions of interestin the three-dimensional data set 380. The detection (at 3354) usesrange cluster scores as a measure to locate flat, vertical surfaces inan image. More specifically, a range cluster score is computed at eachpixel to determine if the pixel lies on a flat, vertical surface. Theflatness of a particular surface is determined by looking at how manypixels are within a given range in a small region of interest. The givenrange is defined by a threshold value that can be adjusted to varyperformance. For example, if a computed range cluster score exceeds aspecified threshold value, the corresponding pixel is marked as adetection. If a corresponding group of pixels meets a specified sizecriteria, the group of pixels is referred to as a region of interest.Regions of interest, for example those regions containing one or moretargets, are determined and passed to a segmenter for furtherprocessing.

Segmentation (at 3356) determines, for each detection of a target 130(shown in FIG. 1 and in FIG. 2), which pixels in a region of interestbelong to the detected target 130 and which belong to the detectedtarget's background. Segmentation (at 3356) identifies possible targets,for example, those whose connected pixels exceed a height thresholdabove the ground plane. More specifically, the segmentation (at 3356)separates target pixels from adjacent ground pixels and the pixels ofnearby objects, such as bushes and trees.

Feature extraction (at 3358) provides information about a segmentation(at 3356) so that the target 130 and its features in that segmentationcan be classified. Features include, for example, orientation, length,width, height, radial features, turret features, and moments. Thefeature extraction (at 3358) also typically compensates for errorsresulting from segmentation (at 3356) and other noise contamination.Feature extraction (at 3358) generally determines a target'sthree-dimensional orientation and size and a target's size. The featureextraction (at 3358) also distinguishes between targets and false alarmsand between different classes of targets.

Classification (at 3360) classifies segmentations to contain particulartargets, usually in a two stage process. First, features such as length,width, height, height variance, height skew, height kurtosis, and radialmeasures are used to initially discard non-target segmentations. Thesegmentations that survive this step are then matched with true targetdata stored in a target database. The data in the target database, forexample, may include length, width, height, average height, hull height,and turret height to classify a target. The classification (at 3360) isperformed using known methods for table look-ups and comparisons.

Data obtained from the segmentation (at 3356), the feature extraction(at 3358), and the classification (at 3360) may, in some embodiments, bedisplayed in one of a variety of user-selectable formats. Typicalformats include a three-view commonly used by armed forces to identifytargets during combat, a north reference plan view, or a rotatedperspective. These display options available to the operator, eitherlocal or remote, are based on the three-dimensional nature of the LADARimage. The results of the feature extraction (at 3358) provide targetinformation including orientation, length, width and height. The targetimage can be displayed from any perspective, independent of the sensorperspective, and the operator can select one of the several displayformats that utilize the adjustable perspective.

The data obtained from the segmentation (at 3356) is then used inidentifying, or “recognizing,” the target. One suitable method for thisidentification is disclosed in:

-   -   U.S. Pat. No. 5,893,085, entitled “Dynamic Fuzzy Logic Process        for Identifying Objects in Three-Dimensional Data,” issued Apr.        6, 1999, to Lockheed Martin Corp. as the assignee of the        inventors Ronald W. Philips, et al.        Other suitable techniques may also be employed. The LADAR system        100 then takes a preprogrammed course of action predicated on        the identity of the target. However, in alternative embodiments,        the data may be presented to a user, typically in a        three-dimensional image. Techniques for this end use are        disclosed in:    -   U.S. Pat. No. 5,644,386, entitled “Visual Recognition System for        LADAR Sensors,” issued Jul. 1, 1997, to Loral Vought Systems        Corp. as assignee of the inventors Gary Kim Jenkins, et al.; and    -   U.S. Pat. No. 5,852,492, entitled “Fused Lasar Range/Intensity        Image Display for a Human Interpretation of Lasar Data,” issued        Dec. 22, 1998, to Lockheed Martin Vought Systems Corp. as the        assignee of the inventors Donald W. Nimblett, et al.        However, this aspect of the invention will be implementation        specific, depending upon the intended end-use of the LADAR        system 100.

Many aspects of the processing by the ATR system 120 aresoftware-implemented, especially the method 3300 in FIG. 33. Someportions of the detailed descriptions herein consequently are presentedin terms of a software implemented process involving symbolicrepresentations of operations on data bits within a memory in acomputing system or a computing device. These descriptions andrepresentations are the means used by those in the art to mosteffectively convey the substance of their work to others skilled in theart. The process and operation require physical manipulations ofphysical quantities. Usually, though not necessarily, these quantitiestake the form of electrical, magnetic, or optical signals capable ofbeing stored, transferred, combined, compared, and otherwisemanipulated. It has proven convenient at times, principally for reasonsof common usage, to refer to these signals as bits, values, elements,symbols, characters, terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantifies. Unlessspecifically stated or otherwise as may be apparent, throughout thepresent disclosure, these descriptions refer to the action and processesof an electronic device, that manipulates and transforms datarepresented as physical (electronic, magnetic, or optical) quantitieswithin some electronic device's storage into other data similarlyrepresented as physical quantities within the storage, or intransmission or display devices. Exemplary of the terms denoting such adescription are, without limitation, the terms “processing,”“computing,” “calculating,” “determining,” “displaying,” and the like.

Note also that the software implemented aspects of the invention aretypically encoded on some form of program storage medium or implementedover some type of transmission medium. The program storage medium may bemagnetic (e.g., a floppy disk or a hard drive) or optical (e.g., acompact disk read only memory, or “CD ROM”), and may be read only orrandom access. Similarly, the transmission medium may be twisted wirepairs, coaxial cable, optical fiber, or some other suitable transmissionmedium known to the art. The invention is not limited by these aspectsof any given implementation.

The operation of this particular implementation of the LADAR transceiver110 in the LADAR system 100 is conceptually illustrated in FIG. 34. Theemitted pulse 125 is transmitted by the optical train (shown in FIG. 6)of the LADAR transceiver 110 on the platform 3410 to scan a geographicalarea called a scan pattern 3420. Each scan pattern 3420 is generated byscanning elevationally, or vertically, several times while scanningazimuthally, or horizontally, once within the field of view 3425 for theplatform 3410 within the field of view 220, shown in FIG. 2. The scanpatterns are sometimes, and will be hereafter herein, referred to as“footprints.” FIG. 34 illustrates a single elevational scan 3430 duringthe azimuthal scan 3440 for one of the footprints 3420. Thus, eachfootprint 3420 is defined by a plurality of elevational scans 3450 suchas the elevational scan 3430 and the azimuthal scan 3440. The principaldifference between the successive footprints 3420 is the location of theplatform 3410 at the start of the scanning process. An overlap 3460between the footprints 3420 is determined by the velocity of theplatform 3410 in the direction of an arrow 3465. The velocity,depression angle of the sensor with respect to the horizon, and totalazimuth scan angle of the LADAR platform 3410 determine the footprint3420 on the ground.

As the LADAR transceiver 110 scans the field of view 220, shown in FIG.2, azimuthally and elevationally as described immediately above, itcontinually emits LASER pulses 125, shown in FIG. 1, in a train. Thetrain of LASER pulses 125 is generated and transmitted as describedabove relative to FIG. 6. Each LASER pulse 125 in the train is fired bya trigger pulse 3500, shown in FIG. 35, at a time t_(fx) in the periodof the clock 410, also shown in FIG. 35.

As described with respect to FIG. 1 and FIG. 2, the LASER pulses 125 arereflected from a target 130 back to the LADAR transceiver 110. Att_(re), as was described relative to FIG. 4, the range gate of the PCE115 is enabled for the period T₁. During the period T₁, the return pulse140, shown in FIG. 1, is received by the LADAR transceiver 110 andcaptured by the capture unit 720, shown in FIG. 7, of the capture andprocess unit 330 of the PCE 115. As described above, this involvescollection of the return pulse 140 as described relative to FIG. 6,detection by the detector array 310 as described relative to FIG. 3 andFIG. 6. The capture unit 720 stores the captured return pulse 140 in theRAM 730, also shown in FIG. 7, in a row-column format.

During the period T1, the AGC 340, shown in FIG. 3 and in FIG. 7, mayautomatically adjust the gain of the post amps 755 and, hence, thedetector array 310. As was discussed relative to FIG. 5A, the AGC 340will compare the intensity of the returned pulse 140 as reflected in theoptical signal 360 to the predetermined value. The AGC 340 will thenincrement the counter 510, shown in FIG. 5A, if the comparison shows theintensity is too low relative to the predetermined value. If the AGC 340increments the counter 510, the gain of the detector array 310 isproportionally increased through the attentuator 550. Note that thisincrease is responsive to the comparison.

Returning to FIG. 35, at time t_(rd), the range gate is disabled, thetime period T₁ terminates, and the time period T₃ commences. The processunit 740, shown in FIG. 7, reads the gated samples stored in the RAM730, and processes them as discussed above to generate the threedimensional data set 380. The period T₃ terminates at t_(ne), at whichtime the read from the RAM 730 is no longer enabled.

However, at t_(ne), the period T₂ begins with the noise gate enable. ThePCE 115 assumes that any “pulse,” e.g., the noise event 3505, detectedduring the period T₂ is generated from noise since it expects all returnpulses 140 to be detected in the period T₁. It is expected that a noiseevent will only be detected relatively rarely. However, when a noiseevent 3505 is detected, the AGC 340 decrements the counter 510, shown inFIG. 5A, as was discussed relative to that figure. Decrementing thecounter 510 reduces the gain of the detector array 310 proportionallythrough the attenuator 550. The period T₂ ends at t_(nd), whereupon thenoise gate is disabled. The process repeats upon the next firing of theLADAR transceiver 110 triggered by the trigger pulse 3500 at t_(fx+1).

Thus, in another aspect, the invention includes a method 3600,illustrated in FIG. 36, for automatically controlling the gain of areceiver in an optical system. The method 3600 includes comparing theintensity of a true return pulse to a predetermined value (at 3610);increasing the gain of an optical detector responsive to the comparison(at 3620); detecting a false returned pulse (at 3630); and decreasingthe gain of the optical detector responsive to the detection (at 3630).Note that the order in which the gain is increased and decreased is notmaterial to the practice of the invention and that the gain will not beadjusted (either up or down) in every operational cycle.

In the context of the illustrated embodiment, the invention furtherincludes a method 3700, illustrated in FIG. 37. The method 3700includes:

-   -   receiving a plurality of true return pulses (at 3705);    -   generating an optical signal including the received true return        pulses (at 3710);    -   capturing the true return pulses in the optical signal (at        3715);    -   processing the captured true return pulses (at 3720);    -   detecting a noise event (at 3725); and    -   automatically controlling the gain in generating the optical        signal responsive to the intensity of the processed return        pulses and responsive to the detected noise event (at 3730).        Again, the order in which the gain is increased and decreased is        not material to the practice of the invention and that the gain        will not be adjusted (either up or down) in every operational        cycle.

This concludes the detailed description. The particular embodimentsdisclosed above are illustrative only, as the invention may be modifiedand practiced in different but equivalent manners apparent to thoseskilled in the art having the benefit of the teachings herein. Forinstance, the LADAR transceiver may, in alternative embodiments, emit acontinuous, as opposed to pulsed, beam. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular embodiments disclosed above may be altered or modified andall such variations are considered within the scope and spirit of theinvention. Accordingly, the protection sought herein is as set forth inthe claims below.

1. An apparatus, comprising: an optical detector; a threshold unitcapable of converting an analog optical signal received by the opticaldetector to a digital representation thereof; a capture unit capable ofcapturing the digital representation of the received optical signal; aprocess unit capable of processing the captured digital representation;and an automatic gain control capable of controlling the gain of theoptical detector responsive to the content of the processed digitalrepresentation, wherein the automatic gain control drives the gainhigher responsive to determining that the intensity of a return pulse inthe optical signal is lower than a target value and drives the gainlower responsive to determining that the intensity of the return pulsein the optical signal is higher than the target value, the automaticgain control including: an intensity median computation circuit capableof comparing the intensity of the return pulse to a predetermined valueand outputting a first signal indicating the result of the comparison;an up/down counter capable of receiving the first signal, incrementingand decrementing responsive to the first signal, and outputting a secondsignal proportional to the count therein; a digital to analog convertercapable of converting the second signal to an analog signal; and anattenuator receiving the analog signal and attenuating a gain signalresponsive to the analog signal.
 2. The apparatus of claim 1, whereinthe optical detector comprises an array of photodiodes.
 3. The apparatusof claim 1, wherein the threshold unit includes a bank of voltagecomparators.
 4. The apparatus of claim 1, wherein the capture unitsamples the digitized representation of the received optical signal andencodes the resultant samples proportionally to the peak of thedigitized representation at the time it is sampled.
 5. The apparatus ofclaim 1, wherein the capture unit includes: a time demultiplexer capableof demultiplexing the digitized representation; and a line encodercapable of encoding the demultiplexed digitized representation.
 6. Theapparatus of claim 1, further comprising a memory in which the captureunit is capable of buffering the captured digital representation andfrom which the process unit is capable of reading the buffered captureddigital representation.
 7. The apparatus of claim 1, wherein the processunit includes: a convolution circuit capable of generating a filteredsignal; and a peak detect circuit capable of detecting the peakamplitude of the filtered signal.
 8. The apparatus of claim 1, furthercomprising a constant false alarm rate circuit capable of detecting anoise event and clamping the content of the up/down counter.
 9. Anapparatus, comprising: an optical detector; a threshold unit capable ofconverting an analog optical signal received by the optical detector toa digital representation thereof; a capture unit capable of capturingthe digital representation of the received optical signal; a processunit capable of processing the captured digital representation; and anautomatic gain control capable of controlling the gain of the opticaldetector responsive to the content of the processed digitalrepresentation, wherein: the automatic gain control drives the gainhigher responsive to determining that the intensity of a return pulse inthe optical signal is lower than a target value and drives the gainlower responsive to determining that the intensity of the return pulsein the optical signal is higher than the target value; and the targetvalue is the median intensity value of the processed return pulses. 10.The apparatus of claim 9, wherein the optical detector comprises anarray of photodiodes.
 11. The apparatus of claim 9, wherein thethreshold unit includes a bank of voltage comparators.
 12. The apparatusof claim 9, wherein the capture unit samples the digitizedrepresentation of the received optical signal and encodes the resultantsamples proportionally to the peak of the digitized representation atthe time it is sampled.
 13. The apparatus of claim 9, wherein thecapture unit includes: a time demultiplexer capable of demultiplexingthe digitized representation; and a line encoder capable of encoding thedemultiplexed digitized representation.
 14. The apparatus of claim 9,further comprising a memory in which the capture unit is capable ofbuffering the captured digital representation and from which the processunit is capable of reading the buffered captured digital representation.15. The apparatus of claim 9, wherein the process unit includes: aconvolution circuit capable of generating a filtered signal; and a peakdetect circuit capable of detecting the peak amplitude of the filteredsignal.
 16. The apparatus of claim 9, further comprising a constantfalse alarm rate circuit capable of detecting a noise event and clampingthe content of the up/down counter.
 17. A method for automaticallycontrolling the gain of a receiver in an optical system, comprisingcomparing the intensity of a returned pulse to a target value, includingcomparing the intensity to a median value of a plurality of returnedpulses; and controlling the gain of an optical detector responsive tothe comparison.
 18. The method of claim 17, wherein controlling the gainof the optical detector includes: incrementing and decrementing anup/down counter; converting an output of the counter proportional to thecontent thereof to an analog signal; and attenuating a power supplysignal proportionally to the amplitude of the analog signal.
 19. Themethod of claim 17, further comprising gating the comparison.
 20. Themethod of claim 19, wherein gating the comparison includes: enabling thecomparison in a time period in which a true return pulse is expected;and disabling the comparison otherwise.
 21. The method of claim 17,further comprising clamping the upper bound of the controlled gain. 22.The method of claim 21, wherein clamping the upper bound includes:detecting a noise event; and decreasing the gain.
 23. The method ofclaim 22, wherein decreasing the gain includes: decrementing an up/downcounter; converting an output of the counter proportional to the contentthereof to an analog signal; and attenuating a power supply signalproportionally to the amplitude of the analog signal.
 24. A method forautomatically controlling the gain of a receiver in an optical system,comprising: comparing the intensity of a returned pulse to a targetvalue; controlling the gain of an optical detector responsive to thecomparison; clamping the upper bound of the controlled gain, including:detecting a noise event; and decreasing the gain; and gating thedetection.
 25. The method of claim 24, wherein gating the comparisonincludes: enabling the detection in a time period in which a falsereturned pulse is expected; and disabling the detection otherwise. 26.The method of claim 24, wherein comparing the intensity of the returnedpulse to the target value includes comparing the intensity to a medianvalue of a plurality of returned pulses.
 27. The method of claim 24,wherein controlling the gain of the optical detector includes:incrementing and decrementing an up/down counter; converting an outputof the counter proportional to the content thereof to an analog signal;and attenuating a power supply signal proportionally to the amplitude ofthe analog signal.
 28. The method of claim 24, further comprising gatingthe comparison.
 29. The method of claim 28, wherein gating thecomparison includes: enabling the comparison in a time period in which atrue return pulse is expected; and disabling the comparison otherwise.30. The method of claim 29, further comprising clamping the upper boundof the controlled gain.
 31. The method of claim 30, wherein clamping theupper bound includes: detecting a noise event; and decreasing the gain.32. The method of claim 31, wherein decreasing the gain includes:decrementing an up/down counter; converting an output of the counterproportional to the content thereof to an analog signal; and attenuatinga power supply signal proportionally to the amplitude of the analogsignal.
 33. A method comprising: receiving a plurality of return pulses;generating an optical signal including the received return pulses;capturing the return pulses in the optical signal; processing thecaptured return pulses; detecting a noise event in the capture of thereturn pulses; and automatically controlling the gain in generating theoptical signal responsive to the intensity of the processed returnpulses and responsive to the detected noise event, including comparingthe intensity of the returned pulses to a target value, whereincomparing the intensity of the returned pulses to the target valueincludes comparing the intensity to a median value of a plurality ofreturned pulses.
 34. The method of claim 33, wherein automaticallycontrolling the gain in generating the optical signal includes:comparing the intensity of each true return pulse to a target value; andadjusting the gain of an optical detector responsive to the comparison.35. The method of claim 33, wherein automatically controlling the gainincludes: incrementing and decrementing an up/down counter responsive tothe comparison; converting an output of the counter proportional to thecontent thereof to an analog signal; and attenuating a power supplysignal proportionally to the amplitude of the analog signal.
 36. Themethod of claim 34, further comprising gating the comparison.
 37. Themethod of claim 33, further comprising clamping the upper bound of thecontrolled gain.
 38. The method of claim 33, further comprising:generating a train of optical pulses; scanning a field of view whiletransmitting the train of optical pulses such that, upon encountering anobject in the field of view, they are reflected as the return pulses.39. The method of claim 33, wherein processing the captured returnpulses generates a three-dimensional data set.
 40. The method of claim39, further comprising identifying a target from the three-dimensionaldata set.
 41. The method of claim 33, wherein generating the opticalsignal includes: detecting the received return pulses; and digitizingthe detected return pulses.
 42. The method of claim 33, whereincapturing the return pulses includes: sampling the optical signal; andencoding the samples of the optical signal.
 43. The method of claim 42,wherein capturing the return pulses includes buffering the encodedsamples.
 44. The method of claim 33, wherein processing the capturedreturn pulses includes filtering the optical signal through a finiteimpulse response filter.
 45. The method of claim 44, further comprisinglinearizing the captured optical signal.